1. Field of the Invention
This invention relates to improved rotary mowers for cutting vegetation, such as grass lawns. More particularly, this invention relates to rotary blade assemblies for rotary mowers that provide an equivalent cut with a substantial reduction in necessary driving power, and modular or gang arrangements of rotary blade assemblies that reduce the necessary driving power of rotary mowers.
2. Description of the Related Art
Many conventional rotary mowers utilize a cutting blade that rotates in a substantially horizontal plane. Commonly, the blade is shaped symmetrically with substantially planar bottom surfaces. Outer portions of the blade are sharpened to provide an edge for cutting grass or other vegetation. An example of such a conventional blade is shown in FIG. 9A. The blade is attached to a drive shaft that rotates. As the mower is moved across a lawn or similar area, the rotating, sharpened edges of the cutting blade cut the vegetation to a width defined by the length of the blade, independently of the length of the sharpened outer end portions.
Most such rotary mowers utilize a motor to provide rotational driving power to the blade. In order to obtain a clean cut, the blade is driven at a relatively high rate of rotation. While varying with the particular dimensions of the cutting blades, a typical manually propelled rotary mower with a twenty-inch cut utilizes an internal combustion motor of 3 to 4 horsepower to provide the driving power required to maintain a high rotational velocity. Most new push mowers and mulchers have increased the size of the driving motors to 5 and even 6 horsepower.
Various adaptations and modifications to this basic design have been proposed. In particular, there have been many prior art attempts to address the problems associated with dulling of the sharpened edges of the cutting member. For example, U.S. Pat. No. 4,779,407 to Pattee describes a rotary member that employs relatively inexpensive, disposable cutting components intended to be frequently replaced. U.S. Pat. No. 5,467,586 to Lin et al. similarly proposes the use of detachably affixed cutting blades. U.S. Pat. No. 4,715,173 to Anderson describes an accessory bar to which are attached various components for cutting, mulching and raking a lawn. Other prior art devices, such as described in U.S. Pat. No. 4,229,933 to Bernard, similarly address the problem of dulling by simplifying the task of removing and reattaching the blade for sharpening.
Many other prior art rotary mowers have been proposed to perform additional tasks beyond the cutting function provided by horizontally disposed cutting surfaces. For example, U.S. Pat. No. 2,720,071 to Watanabe describes a rotary blade having sharpened protrusions that extend into turf to uproot and cut low growing runners. U.S. Pat. No. 2,942,397 to Clark refers to tiller projections that extend downwardly to "power renovate" or "power rake" the soil. U.S. Pat. No. 3,321,026 to Hubbard, U.S. Pat. No. 3,724,182 to Long et al. and U.S. Pat. No. 4,306,407 to Dambroth each similarly utilize downwardly directed times or projections to rake or scarify the soil.
A variety of structures have also been proposed to provide a "mulching" of cut grass. Such devices include U.S. Pat. Nos. 5,167,109 and 5,291,725 to Meinerding that describe additional cutting edges attached to a horizontal blade to finely shred vegetation. U.S. Pat. No. 5,375,400 describes a plurality of triangular-shaped cutting blades extending perpendicularly to a square mounting section to mulch a high volume of cut vegetation. Additionally, the above-noted patent to Anderson describes an accessory to mulch previously cut vegetation.
While improving upon particular aspects of the basic horizontal cutting blade used in rotary mowers, the above-mentioned prior art devices generally share a relatively high power requirement to ensure proper cutting and to perform such additional tasks as mulching, shredding or raking a lawn. This power requirement results from a number of different factors. First, several surfaces of the conventional mower, including the cutting blade, comes into contact with the vegetation. This results in a relatively large amount of friction causing considerable power requirements to rotate the blade and to move the mower forward.
Moreover, due to the forward translation of the mower (Vt), there is both a longitudinal movement of the blade along with a rotational movement of the blade. Since only the outer portions of the conventional blade are sharpened, there is an irregular coverage of the area to be cut inside the cutting path. As seen in FIG. 9B, the outer parts of the cutting path will have many repetitive and overlapping passes in relation to the few necessary passes to cover the center portion of the cutting path.
In order to illustrate the irregular covering of the inside area of the cutting path better, the track of only one of the two outer sharpened edges of the conventional mower blade of FIG. 9A is shown in FIG. 9B. The sharpened edge of the opposite end will have a mirror image path of the shown path (displaced by the length of advance of the mower corresponding to one-half turn), which fills most of the blanks seen in FIG. 9B and further repeats the tracks of the outer ends of the cutting path.
Even after an area of vegetation has been cut by the blade, friction is applied to the blade each time it subsequently scales a previously cut area, causing a further dissipation of energy. Since the cutting blade is substantially horizontal, a relatively large area of the blade rubs against the vegetation. The excess power required because of this effect is proportional to the number of times the blade subsequently scales this previously cut portion, and is also proportional to the radial distance from the center to the cutting edges (i.e., there is a higher torque at the outer ends). Since conventional mowers have blades rotating at large angular velocities in order to ensure a proper cut, the power demand becomes even more pronounced.
Existing mulching mowers work on the basis of repetitive cutting of the clippings while the clippings are in the air (on the fly). According to an Ohio State University Extension Fact Sheet, this is typically facilitated by using "high lift blades with expanded cutting edges and restrictions in the discharge ports" of the mower. These conventional mulching mowers demand extra power and require relatively high tangential speeds and well-sharpened cutters, besides special deck construction, to direct and confine the clippings under the action of the rotating cutters. Further, the rotating cutters of the conventional mulching mowers typically include a plurality of cutters protruding above a horizontal blade.
The increased power requirement associated with prior art mowers results in several drawbacks. For example, it is generally necessary to utilize a relatively large internal combustion motor to obtain a desired level of cutting, mulching, raking, and so forth. The size and weight of such a power source further reduce the handling and ease of use of the mower, and contribute to greater expense in manufacturing, sales (e.g., because of increased storage demand and difficulty in transport), and day-to-day use. In addition to the expense resulting from increased energy consumption, larger internal combustion motors are also characterized by increased exhaust emissions that detrimentally affect the quality of the environment.
In addition, rotary mowers having downwardly protruding cutters according to the present invention (described below) may have certain drawbacks when used to cut grass in uneven terrain. For example, when a rotary mower having a mower blade with downwardly protruding cutters rotating in a horizontal plane traverses a ridge or other surface undulation, the mower blade tends to cut the grass too close to the soil or even gouge into the soil itself. This is because the wheels of a conventional mower, which are positioned outside the rotating path of the blade, provide the only means for regulating the height of cut provided by the mower. Thus, the irregularities in terrain between the wheels of the mower are not taken into account by the mower in determining what height to maintain the cutting blade.